Lithium secondary batteries have features such as high energy density and long service life. Therefore, lithium secondary batteries are widely used as power supplies for electric appliances such as video cameras, portable electronic devices such as laptop computers and mobile telephones, and electric tools such as power tools. Recently, lithium secondary batteries are also applied to large-sized batteries that are mounted in electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like.
A lithium secondary battery is a secondary battery having a structure in which, at the time of charging, lithium begins to dissolve as ions from the positive electrode and moves to the negative electrode to be stored therein, and at the time of discharging, lithium ions return from the negative electrode to the positive electrode, and it is known that the higher energy density of the lithium secondary battery is attributable to the electric potential of the positive electrode material.
Known examples of this kind of positive electrode active material for lithium secondary batteries include lithium transition metal oxides having a layered structure, such as LiCoO2, LiNiO2, and LiMnO2, and spinel-type lithium-manganese-containing composite oxides having a manganese-based spinel structure (Fd-3m), such as LiMn2O4 and LiNi0.5Mn1.5O4.
Since spinel-type lithium manganese-containing composite oxides of this kind are provided at low prices of raw materials, are non-toxic and safe, and have properties of being resistant to over-charging, attention is paid to them as the next-generation positive electrode active material for the large-sized batteries of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. Furthermore, since spinel-type lithium transition metal oxides (LMOs) that are capable of three-dimensionally intercalating and deintercalating of Li ions have superior output characteristics compared with lithium transition metal oxides having a layered structure such as LiCoO2, it is expected to be used in an application where excellent output characteristics are required, such as in batteries for EVs and batteries for HEVs.
Among others, it has been known to have an operating potential at near 5 V by substituting a part of the Mn sites in LiMn2O4 with other transition metals (Cr, Co, Ni, Fe, or Cu). Thus, at present, development of a 5 V-class spinel-type lithium-manganese-containing composite oxide having an operating potential of 4.5 V or more at a metal Li reference potential is being carried out.
For example, Patent Document 1 discloses, as a positive electrode active material for lithium secondary batteries exhibiting an electromotive force of 5 V-class, a high capacity spinel-type lithium-manganese composite oxide positive electrode active material, comprising a spinel-type lithium-manganese composite oxide added with chromium as an essential additive component, and further, nickel or cobalt.
Patent Document 2 discloses a crystal having a spinel structure, LiMn2-y-zNiyMzO4 (wherein M represents at least one selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu, 0.25≤y≤0.6, and 0≤z≤0.1), which performs charging and discharging at a potential of 4.5 V or more with respect to metal Li.
Patent Document 3 discloses, as a positive electrode active material capable of generating an electromotive force of 4.5 V or more and maintaining a discharge capacity, a positive electrode active material for secondary batteries containing a spinel-type lithium-manganese composite oxide represented by a general formula: Lia(MxMn2-x-yAy)O4 (wherein 0.4<x, 0<y, x+y<2, 0<a<1.2; M includes one or more metal elements selected from the group consisting of Ni, Co, Fe, Cr and Cu and contains at least Ni; A includes at least one metal element selected from Si and Ti, and in a case where A includes only Ti, the value of the ratio y of A is 0.1<y).
Patent Document 4 discloses, in a spinel-type lithium-manganese-containing composite oxide having an operating potential of 4.5 V or more at a metal Li reference potential, a spinel-type lithium-manganese-containing composite oxide containing a crystal phase formed by substituting a part of the Mn site in LiMn2O4-δ with Li, a metal element M1 including Ni (M1 is a metal element containing at least one of Ni, Co and Fe), and another metal element M2 (M2 is Ti or a metal element containing Ti and at least one of Mg, Al, Ba, Cr and Nb), wherein the spinel-type lithium-manganese-containing composite oxide contains a composite oxide phase including Ni, Mn and B.
Patent Document 5 discloses a manganese-based spinel-type lithium transition metal oxide represented by Li[NiyMn2-(a+b)-y-zLiaTibMz]O4 (wherein 0≤z≤0.3, 0.3≤y<0.6, and M is at least one or more metal elements selected from the group consisting of Al, Mg, Fe and Co), wherein a>0, b>0, 2-(a+b)-y-z<1.7, and 3≤b/a≤8 in the above formula.